3.1

Free-Space Optical Link

This activity was adapted for high school students taking a class on the science of lasers and light that was taught over a series of five two-hour Saturday lessons at the UC Santa Barbara campus.^6,7 Unlike the previous two activities, this activity as described typically runs for about one hour. In this activity, students build a transmitter circuit that transmits an audio signal, usually music from a student’s or instructor’s mobile phone, over light emitted by a laser or an LED to a receiver circuit that students also build that is connected to a pair of headphones. A schematic of this setup is provided in Fig. 1.

Figure 1.

A schematic of the free-space optical link activity. We provide this schematic to students assembling the link.⁸

To begin the first lesson of the aforementioned class, we introduce the field of photonics and establish the concept of an optical communications link. Then, we provide materials and basic instructions and let students start to put together their optical links. From this point for the duration of the activity, instructors walk around the room to answer questions. We found most of the time required for this activity was taken up in this stage, with students (1) following the directions to build their optical links and (2) troubleshooting their built optical links for errors or a failed part. A photograph of students working on the activity is shown in Fig. 2

Figure 2.

Students in the School for Scientific Thought (SST) Photonics Course working on the free-space optical communication link activity.

Once students build a working optical link, we invite students to place a fiber-optic cable between their laser and photoresistor, beginning a discussion on the benefits of waveguiding the laser light to communications. We also invite students to compare use of a laser with a blue LED as their light emitter. For each light emitter, students compare the quality of transmitted audio to, e.g., the distance between light emitter and photoresistor, or the ability to couple into a fiber-optic cable.

3.2

Color-Mixing activity

We developed an activity around color perception and the difference between additive and subtractive color mixing for science education events attended by middle school students and their families.⁹ The activity is 25-min in length, and self-contained-that is, there is no additional session during which knowledge or intuition is gained before or after the module. Before the activity, the desks of the classroom are arranged such that 2-3 families can cluster around the table. In addition, we put cyan, magenta, and yellow (CMY) dyes mixed in water, paintbrushes, and laminated pages with useful diagrams on the table. We also place posters with more useful diagrams around the classroom. The content of these posters is pictured in Figs. 3a and 3b.

Figure 3.

Handouts provided to students and their families attending the color mixing activity for the purposes of (a) explaining the function of rod and cone cells to human color perception, (b) showing how the colors cyan, magenta, and yellow used in subtractive color mixing relate to red, green, and blue, and (c) challenging students to solve subtractive color mixing problems. Additionally, (d) a diagram of the colored flashlights demonstration at the end of the activity.¹⁰

We begin the activity with a hook: what colors are in the rainbow? This is a question that most young middle-school children (ages 11-14) are able to answer. As we ask this question, we demonstrate a rainbow by shining a white flashlight into a clear glass container filled with water and a compact mirror such that a rainbow forms on the ceiling. We follow up with the questions, what colors are not in the rainbow and how the colors in the rainbow being shown are formed. The main conclusion that students should draw is that the colors in the rainbow are included in white light. Some students are advanced enough to have heard this.

After this introduction, we give a five-minute mini-lecture on red, green, and blue (RGB) receptors in the eye and subtractive color mixing. We explain that by exciting the RGB receptors in different proportions, we can cause the brain to see different colors. For example, as we direct attention to Fig. 3a, we might say to students that while a yellow flower appears yellow because it reflects yellow light, when we see a yellow flower on a screen, it appears yellow because we are turning on tiny red and green lights, or pixels, in different strengths so that the brain interprets the combination as yellow.

This leads into the next segment of the activity which is on color printing (subtractive color-mixing). To motivate this, we draw attention to printer cartridges that we have placed around the room before the activity and ask students why color printers might use CMY and not RGB. We explain that the colors of the CMY color system block RGB colors individually. That is, cyan on a page absorbs red, but reflects blue and green; magenta absorbs green but reflects red and blue; yellow absorbs blue but reflects red and green. Therefore using the CMY color system one can control the amount and color of light that reaches the eye from a page by starting from white and individually reducing red, green, and blue, the opposite process compared to an electronic screen.

At this point, we pass out worksheets and challenge students to make red, green, and blue from the CMY dyes. The worksheet we pass out to students is pictured in Fig. 3c. If students and their families complete this activity quickly, we have an additional worksheet on which they can try to make so-called “secondary” colors such as orange, purple, and brown. During this portion of the activity, graduate student activity leaders circulate around the room to assist families who are completing the activity. One common mistake we see is that students often put cyan in the blue “answer”, and think that the task is complete. It is important to point out the difference between cyan and blue. The blue in the RGB color system is much darker than what many people expect. We have also had instances of students knowing or discovering that they are colorblind. In this case, if the student is open to it, we use what the student has learned to discuss the origin of red-green colorblindness.

We wrap up the activity with a short activity on colored shadows. When it appears that students and their families are nearing completion of the activity, a graduate student volunteer will set up a white sheet (if a plain white wall is unavailable) and red, green, and blue flashlights next to each other with some lateral distance between them. This setup is pictured in Fig. 3d. We dim or turn off the lights in the classroom and turn on the colored flashlights. We ask that a volunteer step in front of the flashlights so that the students discover they see different colored shadows. We use this time to both review the difference between additive and subtractive color mixing while reinforcing the mechanism of subtractive color mixing.

3.3

Gelatin Waveguides

This is another self-contained 25-minute activity that we developed for science education events attended by middle school students and their families.⁹ The setup for this activity is similar: pages with useful diagrams are distributed, and other materials (fiber optic cables, laser pointers, pieces of gelatin) are distributed as needed during the activity. We find it best to hand out materials as they become relevant, so the students do not get distracted during preliminary segments. Before the activity, we set up a demonstration of total internal reflection by mixing water and a few drops of milk or half and half into a clear rectangular container, then testing with a green laser pointer for reflections and visibility of the laser light path through scattering. We also set up a fiber optic cable demonstration by making a few bundles of several fiber optic cables per bundle.

This activity begins with a hook: “What exactly happens when you send a text message or an e-mail?” Usually students volunteer some basic answers related to cellular phone towers, satellites, and sometimes data centers. After allowing some time for discussion, we use the images and illustrations pictured in Fig. 4 that were distributed to families at the beginning of the activity to lead up to explaining that communication on the internet largely relies on lasers, which transmit information in the form of light waves, and a global network of fiber-optic cables, which carry the information.

Figure 4.

Handouts provided to students and their families attending the gelatin waveguides activity for the purpose of explaining the importance of lasers and fiber-optic cables to internet communications.¹¹

After this, we introduce a laser pointer to the classroom and ask the room: “What is special about a laser?” We find it helps to contrast the laser with a conventional flashlight like those that can be found on most mobile phone cameras today. Here, the main takeaways for students are that, unlike flashlights, lasers are very bright, travel in a straight line, and are one color. This is followed by a demonstration of total internal reflection. We motivate this demonstration by asking how to control light over long distances, e.g., to communicate with somebody on another continent. With the room lights off, we point our green laser into the milky water that was set up before the activity and allow students to observe that for sufficiently glancing reflections, all of the light is reflected within the water. This demonstration is pictured in Fig. 5a. We explain that some light is transmitted and some is reflected when it passes between two materials, e.g. air and water, and that if we find an angle where all of the light is reflected within a material, we can make a waveguide, or “light pipe,” out of that material and use it to send the light a long distance.

Figure 5.

Demonstrations in the gelatin waveguides activity: (a) the total internal reflection demonstration used at the beginning of the activity; (b) a schematic of the fiber optic cable demonstration; (c) a partial solution to the challenge we put to participants to bend laser light 180 degrees.

Having explained total internal reflection, we begin a fiber optic cable demonstration by distributing the fiber optic cables at one end of our pre-assembled fiber optic cable bundles, with each family getting one fiber optic cable end, and each activity leader holding all of the fiber optic cables at the other end of the bundle. We explain that activity leaders will send a message through one of the fiber optic cables, without telling families which cable will be used. Activity leaders then send simple Morse code-like signals through the fiber optic cables, and families can observe the signal reaching their cables despite bends and turns in the fiber optic cable bundle. A simple schematic of this demonstration is included in Fig. 5b. At the end of this demonstration, we explain that, as in the tank of milky water, light can be trapped, or guided, in plastic fibers.

In the final part of the activity, we distribute pieces of gelatin (typically 2″x2″x1″) to each student. We challenge the students to form a shape out of their gelatin that can turn a laser beam 180 degrees, and then we pass out laminated flashcards to each student to use as a safe cutting edge. An example partial solution is pictured in Fig. 5c. As in the previous part of the activity, activity leaders walk around the room, answer questions, and guide students as they form their gelatin waveguides.